1. Field of the Invention
The present invention relates to a diphenyl based liquid crystal compound, a liquid crystal composition including the same, and a liquid crystal display including the liquid crystal composition, and more particularly, to a liquid crystal compound having high optical anisotropy and negative dielectric anisotropy, a liquid crystal composition including the same, and a liquid crystal display employing the liquid crystal composition.
2. Description of the Related Art
As the modern industrial society steps into the high information era, electronic displays are regarded as being more and more important for mass distribution of a variety of information. In particular, flat panel displays are highlighted by their advantages of lightweight, thin design, and their application to large size. As a result, flat panel displays are widely used in many fields such as monitors for laptop computers, aircraft controlling equipment, medical instruments, navigators, and measuring instruments. Types of flat panel displays include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and electro luminescent displays (ELDs). In particular, LCDs have been leading the market for the flat panel displays due to their advantages of portability and low power consumption.
The liquid crystal displays (LCDs) are divided into projection LDCs and direct-view LCDs, and the direct-view LCDs are divided into transmissive displays and reflective displays. In transmissive LCDs, the intensity of light emitted from backlight is controlled by LCD panels. In reflective LCDs, images are formed by reflection of natural light and surrounding light onto LCD panels. Among the reflective LCDs, liquid crystals on silicon microdisplays (LCoS) have recently drawn a lot of attentions.
Generally, a LCoS display includes a liquid crystal layer including a liquid crystal material interposed between a silicon substrate and a glass cover. The silicon substrate includes an array of pixels and each of them is conductive and reflective. Although the LCoS display has a screen size of smaller than 1 inch when measured diagonally, it can display a high resolution image. LCoS displays have a small pixel size. Therefore, in general, cell thickness is very small, even smaller than 1 micrometer. Therefore, a liquid crystal composition used in the LCoS display must have a very high optical anisotropy, as opposed to other LCDs in which a low optical anisotropy is usually sufficient.
Liquid crystal compounds used in VA mode LCDs, which use a Vertical Alignment (VA) technique should have high optical anisotropy and negative dielectric anisotropy. The liquid crystal compound must have a low K33/K11 (ratio of elastic constants) and rotation viscosity, and high chemical stability against external influences such as UV, heat, infrared light, air and electric fields. Also the liquid crystal compound must have a liquid crystal phase having a wide temperature range and rapid response speed.
Korean Patent Application No. 99-3372 discloses a liquid crystal composition including a liquid crystal compound, which is a cyclohexene derivative. The liquid crystal composition provides good optical anisotropy and positive dielectric anisotropy. In addition, Korean Patent Application No. 99-3373 also discloses a liquid crystal composition providing good optical anisotropy and positive dielectric anisotropy.
However, a single compound having high optical anisotropy and negative dielectric anisotropy is not well known. Attempts have been made to obtain both high optical anisotropy and negative dielectric anisotropy in a liquid crystal composition by mixing about 5-25 liquid crystal compounds. However, the attempts to manufacture an ideal liquid crystal phase having high optical anisotropy and negative dielectric anisotropy have failed. Accordingly, there is a need to develop a liquid crystal compound having high optical anisotropy and negative dielectric anisotropy and a liquid crystal composition including the compound.